1. Field of the Invention
The present invention relates to an optical scanning device, and an image forming apparatus.
2. Description of the Related Art
Tandem color image forming apparatuses have been known, which form images (visible images) of different colors on a plurality of image carriers and superimpose them one on top of another to form a color image. This image forming apparatus irradiates a write beam, i.e., a light beam corresponding to image information, onto the respective image carriers and scans these beams to form a latent image on the image carriers, and develops the latent images. An optical scanning device that irradiates and scans the write beam generally includes a polygon mirror as a deflector that deflects and scans the write beam from a light source, and a plurality of optical elements (lenses and the like) for imaging the write beam deflected and scanned by the polygon mirror on surfaces of the image carriers. In such an optical scanning device, position and angle change slightly between the respective optical elements due to several reasons including a field curvature characteristic of the optical element, a distortion of a housing of the optical scanning device, deformation of components constituting the optical scanning device due to heat generated by a polygon motor, and a distortion at the time of fitting the image carriers. If a change in position and angle occurs between the respective optical elements, a scanning position of the write beam onto the image carriers changes. Besides, a curvature or inclination of a scanning line by the write beam occurs on the surfaces of the image carriers. As a result, a relative deviation of the scanning position of the write beam, and of the curvature and inclination of the scanning line appear as out-of-color registration. Particularly, out-of-color registration due to the relative deviation of the scanning position in a sub-scanning direction between respective image carriers has been a problem.
In conventional technologies, a pattern image (a registration mark image) is formed for detecting the relative deviation of the scanning position in the sub-scanning direction between respective image carriers, and thereafter, the position is detected by a sensor. Thus, the sub-scanning position is corrected based on the detection result (registration correction).
In the registration correction, however, the pattern image is formed on a transfer medium such as the image carrier and an intermediate transfer belt. If there is a defect or a foreign matter adheres on the image carrier or the intermediate transfer belt, the pattern image may not be accurately formed. As a result, detection cannot be performed, or even if detection is possible, the correction result may not be appropriate. In addition, a sensor that detects the pattern image is arranged near the image carrier or the intermediate transfer belt. Accordingly, the sensor may be stained due to toner scattered from the image carrier or the intermediate transfer belt. In such a case, the pattern image cannot be accurately detected. Furthermore, downtime is required at the time of forming or detecting the pattern image, during which an image forming operation cannot be performed.
Japanese Patent Application Laid-open Nos. 2004-287380, 2000-235290, and Japanese Patent No. 3087748 discloses an image forming apparatus including a beam detector that detects the position of the write beam, in which out-of-color registration is corrected based on the detection result of the beam detector. Thus, by directly detecting the scanning position of the write beam, a pattern image for position detection need not be formed. As a result, such a situation can be reduced that detection cannot be performed, or even if detection is possible, the correction result may not be appropriate. Further, because the pattern image is not formed, the downtime can be reduced at the time of correcting out-of-color registration.
However, when out-of-color registration is corrected by detecting the position of the write beam, the correction is started after a rotation speed (deflection speed) of the polygon mirror reaches that for writing a latent image on the image carriers, and the position of the write beam is detected by the beam detector. As a result, the time for detecting the write beam by the beam detector is required. Therefore, the time until starting correction of out-of-color registration is long.